Rotational coherence beating in molecular oxygen: Coupling between electronic spin and nuclear angular momenta

Area of Science:

• Molecular Spectroscopy
• Quantum Dynamics
• Laser Physics

Background:

• Coherent Anti-Stokes Raman Spectroscopy (CARS) is a powerful technique for probing molecular dynamics.
• Oxygen (O2) exhibits complex rotational structures due to its electronic ground state.
• Accurate thermometry and species concentration measurements require precise understanding of spectral line shapes and dynamics.

Purpose of the Study:

• To investigate time-resolved pure-rotational hybrid femtosecond/picosecond CARS (fs/ps RCARS) of oxygen.
• To analyze coherence beating between unresolved S-branch triplet transitions in O2.
• To improve time-domain models for O2 rotational CARS thermometry.

Main Methods:

• Femtosecond/picosecond hybrid time-resolved pure-rotational CARS experiments on O2.
• Time-domain fitting of RCARS signal intensity to determine transition frequencies.
• Analysis of pressure-dependent dynamics and linewidths.

Main Results:

• Observed coherence beating between O2 S-branch triplet transitions (ΔN=2, ΔJ=2) with frequency separations as low as 480 MHz.
• Determined pressure- and N-dependent O2 linewidths, comparing them to literature values.
• Demonstrated the impact of triplet splitting on spectral evaluations at longer probe delays (>100 ps).

Conclusions:

• The study provides crucial insights into the spectroscopic complexities of O2's electronic ground state.
• The findings enhance the accuracy of time-resolved O2 rotational CARS thermometry and species concentration determination.
• Incorporation of triplet splitting into time-domain models is vital for precise spectral analysis.